As conventional terahertz wave generating methods, Non-Patent Literature 1 discloses three methods, namely an antenna method, a non-linear effect method and a magnetic field application method. In the antenna element method, a voltage bias is applied on a photoconductive antenna that is a microstructure formed on a semiconductor substrate, and in that state ultra-short pulse laser light irradiates the photoconductive antenna, thereby generating terahertz waves. In the non-linear effect method, terahertz waves are generated by irradiating ultra-short pulse laser light on a material having a non-linear receptivity χ (2) on the basis of a light rectification effect. In the magnetic field application method, a magnetic field is applied in parallel on a semiconductor surface, and in that state ultra-short pulse laser light irradiates the semiconductor surface, thereby generating terahertz waves.
Non-Patent Literature 2 discloses a method for tilting the wavefront of the laser light irradiating a non-linear optical crystal with respect to the surface of the non-linear optical crystal as a method of generating Cherenkov radiation in the non-linear optical crystal so as to obtain high-strength terahertz waves. With this method, the wavefront of the laser light is tilted by using a diffraction image transmission system composed of a diffraction grating and lens.
In addition, as one terahertz wave generating method, there is a method of projecting a pump wave onto a nonlinear optical crystal capable of being used in parametric oscillation. FIG. 10 shows the terahertz wave generating principle under this method. With this method, when pulse laser light L is incident on a non-linear optical crystal 100 from a direction orthogonal to the optical axis Z of this non-linear optical crystal 100, a parametric interaction is occurred inside the non-linear optical crystal 100 and a terahertz wave T is generated in a direction A satisfying the non-collinear phase matching conditions. As this terahertz wave generating method, Patent Literature 1 discloses a method using two laser generators. Of these two, one laser generator is a YAG laser that outputs pulse laser light, and the pulse laser light here is set to a pulse width of 15 ns and a wavelength of 1064 nm. The other laser generator is a Yb fiber laser that outputs continuous laser light. The continuous laser light here is used as a terahertz wave injection seeder, and the wavelength is fixed at 1070.2 nm in order to improve terahertz wave strength.